The present invention relates to a hydraulic system, and, more particularly, to a hydraulic system that is suited to perform a clutch operation or variable clutch operation in an automobile automatic transmission.
A hydraulic system is not hermetically sealed, so that air may enter the hydraulic circuit. In general, a hydraulic circuit is a so-called blind circuit which is closed at the hydraulic side, so that a hydraulic fluid can not circulate in the hydraulic circuit and hydraulic actuator. Thus, air which enters the hydraulic circuit and the hydraulic actuator is unlikely to be exhausted out of the circuit, and so it tends to remain therein. Such air in the circuit may delay the response of the hydraulic system and degrade the controllability of the system.
To solve the above-mentioned problem, several techniques have been proposed in which an additional circuit is provided to circulate the hydraulic fluid in the hydraulic circuit in such a way as to exhaust the air out of it, and this is referred to as an air sucking operation herein. As an example of such conventional techniques, Japanese Application Patent Laid-Open Publication No. Hei 10-73105 discloses a hydraulic operation unit which comprises a piston/cylinder having a first chamber and second chamber, and another piston/cylinder having a third chamber. In this operation unit, the first and second chambers are connected via a first pressure valve, and the first and third chambers are connected via a conduit in which a second pressure valve is provided. The first, second, and third chambers have first, second, and third directional control valves, respectively. The valves can control the hydraulic pressure for actuating the piston/cylinders. Those directional control valves are connected to a hydraulic fluid container via a pump. To evacuate the first and third chambers, the first and third directional control valves connect those chambers to the pump. The second directional control valve connects the second chamber to the hydraulic fluid container. Pressure higher than a pressure which is present during normal operation is applied to actuate the first and second pressure valves.
In the above-described hydraulic operation unit, the piston/cylinder may operate during the air sucking operation, so that an accidental operation of the hydraulic actuator during the air sucking operation must be prevented.
It is an object of the present invention to provide a hydraulic system having a reliable air sucking mechanism which is unlikely to malfunction.
To achieve the above mentioned object, the present invention provides a hydraulic system comprising, a hydraulic actuator activated by a supplied hydraulic fluid, a control valve for generating a regulated hydraulic pressure or flow for activating the hydraulic actuator, and a control hydraulic circuit for transmitting the regulated hydraulic pressure or flow generated by the control valve to the hydraulic. The control hydraulic circuit includes an air sucking control valve connected to a primary pressure circuit or regulated pressure circuit, an air sucking hydraulic circuit connecting the air sucking control valve to the hydraulic actuator, and a check valve between the air sucking control valve and the hydraulic actuator, which allows a flow of hydraulic fluid from the air sucking control valve to the hydraulic actuator and prevents a flow of hydraulic fluid from the hydraulic actuator to the air sucking control valve.